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ZIRCON

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Originally appearing in Volume V28, Page 990 of the 1911 Encyclopedia Britannica.
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ZIRCON , a See also:

mineral composed of See also:zirconium silicate, some-times used as a See also:gem-See also:stone. It is believed that the name comes from the Arabic zargun, and is essentially the same as " See also:jar-goon," the name given to certain varieties of zircon. The mineral crystallizes in the tetragonal See also:system, generally in combinations of square prisms and square pyramids, as in See also:figs. i and 2. Zircon is isomorphous with cassiterite and See also:rutile, and like them may occur in geniculated twins. There is no distinct cleavage, and the mineral breaks with a conchoidal fracture.. The hardness is about 7.5. It is notable that the specific gravity has a very wide range, extending from a little below 4 to rather more than 4.7, and being thus greater than that of any other gem-stone. Rarely colourless, zircon is usually See also:brown or red, sometimes See also:orange, yellow or See also:green, and occasionally parti-coloured or zoned. Whilst See also:common zircon is opaque, the gem-varieties are transparent. The dichroism of coloured zircons is always feeble; the See also:double See also:refraction usually strong and of See also:positive sign; and the See also:optical properties of some zircons suggest a biaxial mineral. It was pointed out See also:long ago by . See also:Sir A.

H. See also:

Church that many transparent zircons afford a spectrum marked by certain absorption-bands, a See also:property perhaps due to the presence of See also:uranium. The effect of See also:heat on zircon is remarkable. Most coloured zircons, exposed to a high temperature, either See also:change or lose their See also:colour, but this loss is attended by a gain in brilliancy. The " Matura diamonds " of See also:Ceylon are zircons which have been thus artificially decolorized. Certain zircons when heated in a See also:Bunsen-See also:flame glow with an orange incandescence, whilst others may emit an orange glow when ground on a See also:copper-See also:wheel fed with See also:diamond-dust. Even exposure to sunlight will sometimes modify the colour and lustre of a zircon. Some zircons suffer contraction when heated, so that the specific gravity becomes raised; but the behaviour of zircons in this respect shows such anomalies that S. Stevanovic has been led to suggest the existence of three classes of zircon. One See also:group has a specific gravity of 4.0 and another of 4.7, both remaining unchanged in See also:density when heated. L. J.

See also:

Spencer, who has studied some remarkable crystals from Ceylon, calls the former a-zircon, and the latter 13-zircon. A third class has specific gravity between 4.0 and 4.7, and increases in density on See also:heating. These stones consist, according to Spencer, of an inter-growth of a-zircon or 3-zircon, with a third unstable modification which he distinguishes as -y-zircon, Whilst zircon is usually regarded as a zirconium silicate (ZrSiO4) it is sometimes placed with the oxides as consisting of ZrO2•SiO2. A small proportion of ferric See also:oxide seems to be always See also:present, and to this the colour of zircon, according to G. See also:Spezia, may be ascribed. Traces of so many elements have been recorded in certain zircons that it was at one See also:time See also:pro-posed to See also:call the See also:species polycrasilite from the See also:Greek sroXvs (many) and Kpaars (mixture). Zircon is used as a source of zirconia in various preparations, for incandescent See also:gas-mantles, &c. It was in this mineral that zirconia was originally discovered by M. H. See also:Klaproth in 1789. Zircon See also:fit for use as a gem-stone is often known as " See also:noble " or " See also:precious zircon." The red and orange stones are termed See also:hyacinth (q.v.) and jacinth, whilst those of other See also:colours, as also the colourless transparent zircons, are called See also:jargoon (q.v.). The lyncurium of the ancients, described as an See also:amber-coloured stone used for signets, is supposed by some authorities to have been zircon and by others amber.

The gem varieties of zircon are found in detrital deposits, especially in Ceylon and in New See also:

South See also:Wales, where they accompany See also:sapphire, &c. They occur also in the Anakie sapphire See also:district, near See also:Emerald, in See also:Queensland. A. K. Coomaraswamy has pointed out that most of the stones in the gem-gravels of Ceylon, known locally as toramalli, are zircons rather than tourmalines. Zircon is an See also:accessory constituent of many rocks, especially See also:granite, where it appears to have crystallized at an See also:early See also:stage of consolidation. In microscopic sections, viewed by transmitted See also:light, the zircon by virtue of its high refractive See also:power appears to stand out in See also:relief. It forms an important constituent of the zircon-See also:syenite of See also:Norway. Zircon occurs also in many basic eruptive rocks, notably the basalts of the See also:Rhine and Central See also:France. Being but little subject to alteration, it is common in secondary deposits, as in auriferous and other sands, occurring usually in small characteristic crystals, with rounded angles. See also:Fine crystals of zircon are found in the Ilmen Mountains in See also:Russia, and in See also:Renfrew co., See also:Ontario, where it occurs in crystalline See also:limestone. Many localities in the See also:United States yield zircon, especially in New See also:York See also:state and in See also:North Carolina: it has been largely worked in See also:Henderson co..

N.C. Zircon occurs also in See also:

Tasmania. Certain varieties of zircon have received distinctive names, such as the azorite, which occurs in sanidine-See also:trachyte in the See also:Azores. Several other minerals seem to be altered zircon, generally hydrated, such as malacon, cyrtolite and oerstedite, the last being a See also:Norwegian mineral containing See also:titanium and See also:magnesium. Auerbachite is a See also:Russian mineral closely related to zircon. (F. W.

End of Article: ZIRCON

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